Many newer aircraft make extensive use of composite materials to reduce weight. Such composite panels for the skin of aircraft are susceptible to breakage caused by bird strikes, hail, and other debris. In conventional metal panels, such damage is typically readily visible as dents. However, pilots and maintenance crews may not detect the damage of composite panels as readily since composite panels typically spring-back and do not dent. In addition, cracks to composite panels can be difficult to visually detect. This allows the possibility of reduced performance or later catastrophic failure due to an undetected breakage. As such, examining an aircraft made of composite panels for damage is both time consuming and unreliable.
Some conventional techniques for detecting damage in composite materials include several examples of using optical fiber embedded in the composite material. Techniques for using the fiber as a sensor include embedding Bragg cells at intervals along the fiber to reflect laser pulses and laying individual fibers in a grid pattern each terminated by a mirrored surface to isolate damage to a particular row and column of fiber. The former has a granularity dependent upon the spacing of the Bragg cells and includes the extra cost of forming them. The latter has a granularity set by the grid density and will only detect situations in which the damage is severe enough to sever the fiber.